A system and related methods for heating, cooling, and dehumidifying air is disclosed. In an embodiment, the system includes an indoor unit having a first coil assembly and a second coil assembly. When the system is operating in a cooling mode or a heating mode, the first coil and second coil are in parallel fluid communication. When the system is in a dehumidifying mode, the first coil and second coil are in serial fluid communication, which enables the first coil to function as a condenser and the second coil to function as an evaporator. In an embodiment, the system includes an outdoor unit, such as a heat pump or an air conditioning condensing unit. The outdoor unit includes a heat exchanger fan responsive to dehumidifying mode by reducing fan speed, or deactivating the fan entirely. The disclosed system provides negligible or no change in sensible heat while providing dehumidification.